1. Field of the Invention
This invention relates to improvements in a high frequency discharge lamp for a spectral-line source which can be effectively used as a light source in, for example, spectroscopic measurements etc.
2. Description of the Prior Art
A feature in the high frequency discharge is that the multiplication of electrons takes place and the discharge starts merely in such a way that electrons accelerated by an electric field ionize gaseous molecules (atoms) by collision (even when positive ions exist, they hardly move because their masses are great and the duration of the electric field in one direction is short). That is, in the d.c. or low frequency discharge, the generation of charged particles based on the collision between charged particles and an electrode or the generation of electrons (thermions) from an electrode is an important factor, whereas in the high frequency discharge, the emission of electrons from an electrode is no required. Since a spectrum owing to such high frequency discharge is produced from a comparatively large space, it has the merits of a high spectral-line intensity and a very narrow spectral-line width. It is therefore suitable for uses as light sources in the atomic absorption spectroscopy, the atomic fluorescence spectroscopy etc.
FIG. 1 shows the schematic construction of a high-frequency electrodeless discharge lamp which has hitherto been employed as a kind of spectral light source exploiting the high frequency discharge. A discharge envelope 3 which is made of an electrically-insulating and light-transmitting material such as quartz is arranged within a space 10 surrounded by a wall of a heat insulating material 2 provided inside a cylindrical metallic container serving as a high-frequency shield 1. The discharge envelope 3 need not be wholly formed of light transmitting material, but only a part thereof (end wall part 3a in the figure) may well be formed of light transmitting material (material capable of transmitting spectral lines).
Contained in the discharge envelope 3 are a filling substance 4 and a firing rare gas which is easily ionized and which is stable. The filling substance 4 is made up of a substance contributive to the light emission, that is, a simple substance element which generates the desired spectral lines and/or a halide, for example, of the element which vaporizes more easily than in the case of the simple substance.
A coil 5 through which a high frequency current flows is disposed around the discharge envelope 3. The high frequency output of a high frequency power source 30 whose output frequency is higher than about 1 MHz is led to a connector 6 by a coaxial cable 31. The high frequency power is subsequently fed to a matching circuit 7 through lead wires 6a and 12, and after passing through the matching circuit 7, it is supplied to the coil 5 through lead wires 8 and 8'. A high-frequency magnetic field is produced in the internal space 11 of the discharge envelope 3 by the high frequency current flowing through the coil 5. An induces electric field of a magnitude proportional to the time variation of the high-frequency magnetic field is generated, to cause the high frequency discharge in the firing rare gas. The interior of the discharge envelope is heated by this high frequency discharge, and the filling substance 4 is volatilized. (There is also a method wherein an external heating coil is separately provided, by which the filling substance 4 is heated and volatilized.)
The molecules of the filling substance 4 volatilized into the discharge space 11 are excited by colliding with electrons generated by the high frequency discharge, and radiate spectral lines inherent to the molecules. Otherwise, the molecules are dissociated by colliding with the electrons, atoms produced by the dissociation are excited by collision with the electrons, and the atoms radiate spectral lines inherent to the element. The spectral lines are transmitted through the end wall 3a of the discharge envelope 3. Subsequently, they are taken out as shown by an arrow 13 through an observation window 9 which is provided in the metallic container 1.
In case of employing the prior-art high-frequency electrodeless discharge lamp stated here, there are problems as described below. Firstly, unless the inside diameter of the metallic container 1 is sufficiently large as compared with the outside diameter of the coil 5 for passing the high frequency current therethrough, the electromagnetic coupling between the coil 5 and the shielding metallic container 1 becomes significant, and electric power which is actually used for the high frequency discharge decreases. Usually, the inside diameter of the metallic container 1 is made 2 to 3 or more times greater than the outside diameter of the coil 5. This incurs the disadvantage that the whole light source cannot be constructed to have a small size. Secondly, the electric field intensity necessary for starting the high frequency discharge is much higher than the electric field intensity required for maintaining the discharge after having been once started. Therefore, unless a high frequency power source of high power is used, the required high frequency discharge cannot be started. This incurs the disadvantage that the whole apparatus including the high frequency power source beomes large-sized and expensive.
To the end of eliminating the disadvantages in the case of employing the prior-art high-frequency electrodeless discharge lamp set forth above, there have been already proposed improvements in the high-frequency electrodeless discharge lamp as disclosed in Japanese Patent Application Public-disclosure No. 41572/1973. In the proposal, in order to supply the high-frequency electric field into the discharge envelope on the principle of a coaxial waveguide (or coaxial cable), the discharge envelope is placed on the fore end of the inner conductor of the coaxial waveguide inside the outer conductor thereof. In addition, thermal radiation means for heating and volatilizing the filling substance in the discharge envelope is provided outside the outer conductor. In this case, the outer conductor which surrounds the periphery of the discharge envelope acts as a resonator for high frequencies. Accordingly, the high-frequency electric field is intensely concentrated on the fore end part of the inner conductor, and it is also formed within the discharge envelope placed on the fore end of the inner conductor through the resonating outer conductor, whereby the vapor of the filling substance is excited and caused to emit light. In the discharge lamp according to the proposal, however, the discharge envelope is placed on the fore end part of the inner conductor on which the high-frequency electric field is intensely concentrated, so that the portion of quartz glass forming the discharge envelope which lies in contact with the fore end part of the inner conductor is rapidly deteriorated and that the discharge envelope is destroyed when used for about 24 hours.